Converter circuits are used in a multiplicity of applications. One converter circuit whose voltage can be scaled easily is specified in WO 2007/023064 A1. FIG. 1 illustrates a converter circuit such as the converter circuit disclosed in WO 2007/023064 A1, although, for the sake of clarity, FIG. 1 illustrates only one phase module of the converter circuit. The converter circuit therein has one phase module for each phase, where each phase module includes a first and a second sub-converter system, and the sub-converter systems are connected in series with one another. The junction point between the two series-connected sub-converter systems forms an outlet connection, for example, for an electrical load. Each sub-converter system includes at least one two-pole switching cell. The switching cells are connected in series with one another when there are a plurality of switching cells in one sub-converter system. Each two-pole switching cell has controllable bidirectional power semiconductor switches with a controlled unidirectional current-flow direction, and a capacitive energy store. In FIG. 1, each switching cell has two series-connected controllable bidirectional power semiconductor switches with a controlled unidirectional current-flow direction, and a capacitive energy store connected in parallel with the series circuit by the power semiconductor switches. A converter circuit of this generic type is also specified in WO 2007/33852 A2.
Since the converter circuit according to WO 2007/023064 A1 or according to WO 2007/33852 A2 contains weakly damped resonant circuits, consisting of two or more phase modules, the oscillations which occur therein must be damped for control-engineering purposes in the currents through the first and the second sub-converter systems. In this context, WO 2007/33852 A2 specifies a control method which is based on the principle of freely selectable time intervals for switching operations of the controllable bidirectional power semiconductor switches in the switching cells in the first and second sub-converter systems.